1. Field of the Invention
The present invention relates to an electronic endoscope system, or more particularly, to an electronic endoscope system in which a plurality of types of endoscopes that are different from one another in terms of specifications or usage is employed and an object image is viewed through a monitor.
2. Description of the Related Art
Endoscopes that permit observation of an intracorporeal region that is invisible with naked eyes are, as already known, widely used for diagnosis or cure in the field of medicine. In recent years, an electronic endoscope that includes a CCD or any other solid-state imaging device, which converts an object image into an electric signal, and that permits observation of an object by means of a monitor has become popular. As the electronic endoscope, various endoscopes are adopted depending on a region to be observed. The endoscope is connected to a light source device and a camera controller (signal processing unit) including a signal processing circuit. Moreover, the signal processing circuit includes an image processing circuit for the purpose of improving image quality or enhancing an object image. In order to improve a contrast, for example, a symmetrical two-dimensional digital filter like the one having a matrix presented below is employed. The matrix presented below indicates coefficients to be applied to a center pixel and surrounding pixels.
−1 −5 −1
−5 25 −5
−1 −5 −1
On the other hand, a fixed focus optical system is generally adopted as an optical system for endoscopes because of its simplicity and excellent maneuverability. The fixed focus optical system is designed so that an endoscope can offer a required depth of field for each region to be observed.
However, when the fixed focus optical system is used to extend a depth of field, an f-number dependent on the optical system must be increased. This poses a problem in that brightness is degraded. Moreover, diffraction effects impose a theoretical limit on the depth of field. The depth of field cannot therefore be extended infinitely.
In contrast, a technique for extending a depth of field to be offered by an optical system has been disclosed in, for example, U.S. Pat. No. 5,748,371 or “Extended depth of field through wave-front coding” written by Edward R. Dowski, Jr. and W. Thomas Cathey (Appl. Opt., Vol. 34, 1859–1866, 1995). FIG. 22 schematically shows an extended depth-of-field optical system in accordance with a related art. An endoscope system in which the above technique is implemented includes: as shown in FIG. 22, an imaging means 104 such as a CCD; a cubic phase-modulation mask 102 located at the position of an exit pupil of an optical system that is a system of lenses 103 that converges an image of an object 101 on the light receiving surface of the imaging means 104; and an image processing unit 105 that constructs an image on the basis of image data produced by the imaging means 104.
One side of the cubic phase-modulation mask 102 is a plane, and the other side thereof has, as shown in FIG. 23, a shape expressed with Z=A(X3+Y3). FIG. 23 is an explanatory diagram showing the appearance of the cubic phase-modulation mask 102. A denotes any coefficient. Specifically, one side of the cubic phase-modulation mask 102 is a plane contained in an XY plane, and the other side is a surface of third order that satisfies the above expression in the direction of a Z axis orthogonal to the XY plane. FIG. 23 is an explanatory diagram showing the surface of third order within a range from X=−1 and Y=−1 to X=+1 and Y=+1. Consequently, the surface of third order varies depending on the coefficient A.
The cubic phase-modulation mask 102 gives a phase shift expressed as P(X, Y)=exp(jα(X3+Y3)) to light passing through the mask. Herein, the coefficient α is preferably much larger than 20. Consequently, a response derived from an optical transfer function (OTF) is equal to or smaller than 0.2. The size of a point image affected by aberration that brings about a rotationally asymmetrical point image is much larger than the size of a pixel location in the imaging means 104.
In case of an ordinary optical system not having the cubic phase-modulation mask 102, a response derived from an optical transfer function varies from the one graphically shown in FIG. 24 to the one graphically shown in FIG. 25 as the object 101 is deviated from an in-focus position. If the object 101 is further deviated from the in-focus position, the response varies from the one graphically shown in FIG. 25 to the one graphically shown in FIG. 26. FIG. 24 is a graph showing a response that is derived from an optical transfer function (OTF) characterizing an ordinary optical system with the object located at the in-focus position. FIG. 25 is a graph showing a response that is derived from the optical transfer function characterizing the ordinary optical system with the object deviated from the in-focus position. FIG. 26 is a graph showing a response that is derived from the optical transfer function characterizing the ordinary optical system with the object further deviated from the in-focus position.
In the case of the extended depth-of-field optical system having the cubic phase-modulation mask 102, the variation of the response derived from the optical transfer function dependent on the deviation of the object is discernible from FIG. 27 to FIG. 29. Even when the objects lies at the in-focus position, the response derived from the optical transfer function deteriorates abruptly. However, the variation of the response dependent on the deviation of the object from the in-focus position is limited. FIG. 27 is a graph showing a response that is derived from the optical transfer function characterizing the extended depth-of-field optical system with the object located at the in-focus position. FIG. 28 is a graph showing a response that is derived from the optical transfer function characterizing the extended depth-of-field optical system with the object deviated from the in-focus position. FIG. 29 is a graph showing a response that is derived from the optical transfer function characterizing the extended depth-of-field optical system with the object further deviated from the in-focus position.
An image converged by the optical system is passed through an inverse filter that is characterized by the reverse of the (OTF) characterizing the cubic phase-modulation mask 102 shown in FIG. 30 and that is included in the image processing unit 105. Consequently, the optical transfer functions graphically shown FIG. 27 to FIG. 29 are changed to those graphically shown in FIG. 31 to FIG. 33. FIG. 30 is a graph showing the characteristic of the inverse filter that acts on the response derived from the optical transfer function characterizing the an extended depth-of-field optical system. FIG. 31 is a graph showing a response derived from an optical transfer function (OTF) obtained by reflecting the characteristic shown in FIG. 30 of the inverse filter on the optical transfer function shown in FIG. 27. FIG. 32 is a graph showing a response that is deviated from an optical transfer function (OTF) obtained by reflecting the characteristic shown in FIG. 30 of the inverse filter on the optical transfer function (OTF) shown in FIG. 28. FIG. 33 is a graph showing a response that is derived from an optical transfer function (OTF) obtained by reflecting the characteristic shown in FIG. 30 of the inverse filter on the optical transfer function (OTF) shown in FIG. 29.
The responses derived from the optical transfer functions shown in FIG. 31 to FIG. 33 are analogous to the response derived from the optical transfer function characterizing the ordinary optical system with the object located at the in-focus position. The inverse filter is, for example, an asymmetrical two-dimensional digital filter having a matrix presented below. The matrix presented below lists coefficients that are applied to a center pixel and surrounding pixels.
400 −300 −40 −20 −20
−300 225 30 15 15
−40 30 4 2 2
−20 15 2 1 1
−20 15 2 1 1
As the ordinary optical system goes out of focus, a blur stemming from the fact occurs.
As the extended-depth-of-field optical system goes out of focus, an image produced by the imaging means 104, that is, an unprocessed image is blurred. This is because of aberration that attributes from the cubic phase-modulation mask 102 and that brings about a rotationally asymmetrical point image. The degree of aberration or blurring is nearly constant. When the image is processed using the aforesaid inverse filter, an image produced is less affected by the fact that the optical system is out of focus and similar to an image converged by the ordinary optical. Consequently, the system shown in FIG. 22 offers an extended depth of focus.
Japanese Unexamined Patent Application Publication No. 2000-5127 discloses an endoscope system to which the aforesaid system configuration is adapted. The disclosed endoscope system includes a plurality of types of endoscopes and permits, as shown in FIG. 34, viewing of an object image through a monitor 116. Among the plurality of types of endoscopes, at least one endoscope 111 has an optical phase modulation mask 113 such as a cubic phase-modulation mask included in an optical system 112. Furthermore, the endoscope 111 has an optical transfer function restoring means 115, which is mated with the optical phase modulation mask 113, installed as an output stage of an imaging device 114.
The optical transfer function restoring means 115 must include a restoring means that is equivalent to an inverse filter and that is mated with the optical phase modulation mask 113 included in the optical system 112. The optical transfer function restoring means 115 may be, as shown in FIG. 34, incorporated in the endoscope 111 or may be incorporated in a camera controller (signal processing unit) 117 which displays an image on the monitor 116 and to which the endoscope 111 is connected. A light source device 118 is also included. Owing to the system configuration, even when any of various endoscopes is employed, an extended depth of field can be offered and an image enjoying a high-resolution can be produced irrespective of the type of optical phase modulation mask 113 or the presence or absence thereof.
Furthermore, Japanese Unexamined Patent Application Publication 2000-266979 discloses a means for mounting an optical phase modulation mask in an objective optical system included in an endoscope. Herein, the optical phase modulation mask to be mounted in the optical system is a rotationally asymmetrical optical element such as a cubic phase-modulation mask intended to help the optical system offer an extended depth of field. Included aside from the rotationally asymmetrical optical element are an aperture stop whose aperture is rotationally asymmetrical and a means for positioning the optical element in a direction of rotation, in which the optical element is rotated about an optical axis, with respect to a solid-state imaging device. Owing to these components, the position of the rotationally asymmetrical optical element in the direction of rotation can be determined accurately. Thus, the direction of a rotationally asymmetrical blur is determined so that a blurred image can be restored to an accurate image through image processing.
However, U.S. Pat. No. 5,748,371 and Japanese Unexamined Patent Application Publication No. 2000-5127 describe that: when the technology for offering an extended depth of field by including an optical phase modulation mask in an optical system is implemented in an endoscope, an optical transfer function restoring means is needed for restoring an optical transfer function deteriorated due to the inclusion of the optical phase modulation mask and producing a high-resolution image. Consequently, restoring means must be included in an image processing circuit incorporated in a camera controller (signal processing unit) or an endoscope in one-to-one correspondence with optical phase modulation masks.
The image processing circuit incorporated in the camera controller included in a general endoscope system has the ability to adjust the visibility of an image, which is produced using an imaging optical system, by enhancing image signal components, which fall within a specific spatial frequency band, according to a response derived from an optical transfer function characterizing an imaging optical system. However, the image processing circuit does not include a restoring means that is mated with an optical phase modulation mask included in the optical system in an endoscope and that is intended to help the optical system offer an extended depth of field. Therefore, when an endoscope having an optical phase modulation mask included in an optical system is employed, a resolved image cannot be produced. Thus, the endoscope system cannot guarantee interchangeability between the endoscope having the optical phase modulation mask and an endoscope not having it.
Moreover, an optical transfer function restoring means may be incorporated in an endoscope in order to guarantee the interchangeability. In this case, an A/D converter for analog-to-digital converting an image, a signal converter for converting a resultant digital signal into a video signal, an image processor for restoring an optical transfer function, a signal converter for converting the video signal into the image signal, and a D/A converter must be incorporated in the endoscope. The resultant circuitry is complex and large in size. This invites a great increase in the size of a main body of the endoscope and deteriorates maneuverability thereof.
Furthermore, in order to produce a high-resolution image, optical phase modulation masks and optical transfer function restoring means must be included in one-to-one correspondence. For example, when a rotationally asymmetrical optical element is adopted as an optical phase modulation mask, if an error occurs during assembling during which the optical element is disposed to rotate about an optical axis, the optical element cannot be mated with an optical transfer function restoring means. This poses a problem in that a high-resolution image cannot be produced. As a solution, Japanese Unexamined Patent Application Publication No. 2000-266979 discloses a means for accurately positioning the rotationally asymmetrical optical element in a direction of rotation. However, this solution has a drawback that the inclusion of the means leads to a complex imaging optical system.